Up to the present time, the known mechanisms for converting linear motion to rotary motion have had one or more serious disadvantages. Examples of such devices are a ratchet and pawl movement, wobble plate, walking beam, Scotch yoke and the most common being a shaft having offset crank throws with connecting rods pivoted directly to a piston or to a crosshead. Most of these have inherent variable thrust in one revolution due to angularity of the connecting rod or its equivalent part. The thrust or torque actually imparted to the driven shaft follows a sine curve of harmonic motion for each piston that is connected to the crankshaft.
In addition to the sine curve representing transmitted torque from the piston to the crankshaft due to the angularity of the connecting mechanism, there is the sinusoidal shape of the piston speed curve. The expander described herein has a reciprocating piston so that no torque is transmitted at the point of piston travel reversal, but the compensation for this characteristic is well known and may take the form of a flywheel and/or multi cylinders arranged at 45.degree., 60.degree., 90.degree. or 180.degree. to each other.
The present invention does, however, eliminate all thrust variation by completely eliminating the angularity of the connecting means between the piston and the shaft. Vector resolution of the force dependent upon angularity is therefore unnecessary.
While history reveals that the reaction turbine may have been the first prime mover completely independent of natural forces, the piston type expander has inherent advantages not present in the turbine which is a constant load, constant speed power source. The external combustion power plant utilizing a Rankine, Cornish, Carnot or Clausius cycle has a tremendous advantage in that it produces maximum torque at zero speed. In addition, it is fundamentally a variable speed prime mover adapted especially to the wide range of power and speed requirements of the modern land vehicle whether used on rails, roads or off the highway. Furthermore, the ability of the external combustion power plant to use nearly any type of fuel to generate the elevated vapor (steam)pressure ranges from sawdust to nuclear fission. The internal combustion motor is limited to a relatively small and dwindling supply of petroleum extract or a gas which must be produced on the vehicle.